1. Field of the Invention
The present invention relates to an apparatus for an ion measurement system including means for monitoring the system, and more particularly to apparatus and method for sensing failures in specific ion electrodes and reference electrodes.
2. Description of the Prior Art
The concentration of certain ions is an important parameter in many industrial and medical systems. Measurements of both cations and anions, including sodium, chloride, potassium, nitrate, calcium, ammonia, sulfur dioxide, and hydrogen, may be provided by the use of an ion-selective electrode combined with a reference electrode. The ion electrode and the reference electrode are immersed in a sample of a solution containing the ions of interest and provides a real time direct analytical quantitative measurement of the ion concentration therein. The ion concentration information appears as an electrical potential produced by the ion electrode in combination with the reference electrode operating at a constant potential. Typically, the electrical potential is in the millivolt range. For example, the hydrogen ion measurement system provides a potential of 59.16 mV per decade change of the hydrogen ion activity. In many processes, a continuous measurement is performed, and the process may include alarm systems to indicate when the ion concentration is outside of predetermined limits.
There are a number of problems which can occur with such measurement systems which may result in an undetected failure of the ion electrode or the reference electrode. In many instances, such undetected failures will have catastrophic results. For example, in medical systems, such as kidney dialysis processes, a failure may result in a continuous pH reading of 7.0 while the solution being measured is out of limits. Such failure can and does result in serious damage or death to a patient. In industrial processes, severe economic damages may result from faults, undetected failures, and degradation of ion sensing electrodes such as coating of the electrode by contaminates in the process stream, and physical failure of the electrode such as cracked glass and open leads.
There have been attempts in the prior art to provide means for monitoring ion measurement systems. For example, Blackmer in U.S. Pat. No. 3,661,748 discloses a device in which an ac signal is applied via an electrode to the conductor fluid in which the electrode system is disposed. An ac signal detector is connected to the dc circuitry to measure the ac current flow. A threshold circuit responsive to the output of the ac signal detector indicates fault in the electrochemical sensor system when the output is of a predetermined magnitude. Blackmer utilizes an ac signal level in the 100 millivolt range which is on the same order of magnitude as the dc signals to be measured by the system. In some instances, this large magnitude signal may affect the solution being measured. This system measures a change in resistance of the electrode membrane by a phase detector using the ac signal source as a phase reference. A resistance threshold is provided such that an alarm will sound when the threshold is exceeded. Connery et al, in U.S. Pat. No. 4,189,367, teach a continuous test of an ion selective electrode by sending a dc test current through the electrode system and measuring the voltage change produced. This system measures the total specific ion electrode impedance but does not provide any method of detecting the impedance of the reference electrode or contamination of the reference electrode from the process stream. In the Connery et al system, the procedure periodically applies a dc potential in one polarity and thereafter a second potential in the opposite polarity. A microprocessor analyzes the resulting voltages to determine if any damage has occurred to the ion selective electrode. McAdam et al, U.S. Pat. No. 4,168,220, show a method for detecting the fouling of a membrane cover of an electrical cell by comparing the current output from the cell in its normal mode to the output of a substitute electrode.
None of the above mentioned prior art patents disclose a procedure for monitoring of both the reference cell and the ion selective cell continuously for contamination and for physical failure of the cells. Thus, there is a need for an ion measurement system including a continuous monitoring system which will not affect the measurements but which will show when any change occurs in the measurement system not occasioned by a process change.